1. Technical Field
The present invention relates to organic electroluminescent (EL) devices used for displays, light sources, and the like. The present invention particularly relates to organic EL compounds suitable for coating-type organic EL devices and also relates to an organic EL device containing the organic EL compounds.
2. Related Art
In recent years, organic EL devices including organic EL elements have been being developed. The organic EL devices are suitable for light-emitting displays alternative to liquid crystal displays. The following documents disclose the organic EL devices and methods for manufacturing the organic EL devices: JP-A-2000-323276 (hereinafter referred to as Patent Document 1), JP-A-2002-536492 (hereinafter referred to as Patent Document 2), JP-A-6-73374 (hereinafter referred to as Patent Document 3), JP-A-6-33048 (hereinafter referred to as Patent Document 4), JP-A-2005-285466 (hereinafter referred to as Patent Document 5), and Japanese Patent No. 3290432 (hereinafter referred to as Patent Document 6).
A known technique useful in manufacturing a conventional organic EL device is as follows: a host material and a luminescent dopant are used to form a light-emitting layer such that the conventional organic EL device has high light-emitting efficiency and long life and the light emitted from the conventional organic EL device can be varied in color. The technique is widely used for organic EL devices manufactured by the vapor deposition of an organic material but is not commonly used for coating-type organic EL devices manufactured by forming a layer of a polymeric material by an ink jet process (a droplet ejection process) or a spin-coating process.
The meanings and features of the host material and the luminescent dopant are as described below.
(1) The host material is a material that allows both electrons and holes to flow therethrough.
(2) In an organic EL device including a light-emitting layer containing no luminescent dopant but the host material, light is emitted from the host material. However, in an organic EL device including a light-emitting layer containing the luminescent dopant and the host material, light is not emitted from the host material but is emitted from the luminescent dopant.(3) The spectrum of the light emitted from the organic EL device containing the luminescent dopant and the host material corresponds to that of the fluorescent or phosphorescent light emitted from a luminescent center in the luminescent dopant. The term “luminescent center” herein means a portion of the luminescent dopant or an organic group that can emit strong fluorescent or phosphorescent light. The spectrum of the light emitted from the organic EL device depends on the luminescent center.
Problems listed in Table 1 below are probably reasons why the host material and the luminescent dopant are not used for the coating-type organic EL devices.
(I) The luminescent dopant bleeds (precipitates or segregates) from a coating formed from a solution containing the host material and the luminescent dopant. This is due to a phenomenon that after a solution containing a polymer and a low-molecular-weight substance is applied onto a member and then dried, the low-molecular-weight substance bleeds out of the surface of a coating formed from this solution or segregates in the coating (see Conventional Techniques 1 and 3 in Table 1).(II) The luminescent dopant has a low ability to trap holes and electrons. This is particularly serious when the host material is a conjugated polymer. Since the holes and the electrons primarily flow through such a conjugated polymer, the luminescent dopant can hardly trap the holes or the electrons (see Conventional Techniques 1 and 2 in Table 1.(III) The development of materials serving as a luminescent dopant has been delayed. This is because in the field of coating-type organic EL devices, the host material-luminescent dopant system is less effective than depositional materials (low-molecular-weight electroluminescent materials) because of the above two reasons.
TABLE 1Host materialLuminescent dopantMolecularConjugated orMolecularConjugated orweightnon-conjugatedweightnon-conjugatedConventionalHighConjugatedLow—Technique 1ConventionalHighConjugatedHighConjugatedTechnique 2(oligomer)ConventionalHighNon-conjugatedLow—Technique 3PresentHighNon-conjugatedHighConjugatedInvention(oligomer)